Apple today released the fourth developer beta of iOS 12.4. The update doesn’t yet include any visible changes for users, but it’s likely laying the groundwork for future changes.
Step 1 Connect your iDevice to PC with an original USB cable and launch 3uTools.
Step 2 Go to Flash & JB → Easy Flash, select firmware iOS 12.4beta4. If you want to upgrade with data, please check the box "Retain User's Data While Flashing". There will be a pop reminder after you click "Flash", please read it with caution.
You are recommended to back up your iDevice regularly in case of data loss.